1. Field of the Invention
The present invention relates to electroluminescent display devices. In particular, the present invention relates to a full-color electroluminescent display device and a method of fabricating the same having a structure providing enhanced blue, green, and red color purity and luminescent efficiency.
2. Description of the Related Art
An electroluminescent (EL) display device is a display device where voltage may be employed in light emitting layers to combine electrons and holes to form images. EL display devices have superior characteristics as compared to other display devices, such as excellent visibility, light weight, wide viewing angle, high color purity, and relatively low power consumption.
An EL display device may include a substrate, a light emitting diode having two electrodes, i.e., pixel electrode and counter electrode, and at least one light-emitting layer. The light-emitting layer may include a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL) arranged sequentially between the pixel electrode and the counter electrode. When voltage is applied between the pixel electrode and the counter electrode, holes may be emitted from the pixel electrode through the HIL and the HTL into the EML, and electrons may be emitted from the counter electrode through the EIL and the ETL into the EML. The holes and electrons may recombine in the EML to generate excitons that emit light, which may be evaluated in terms of its color and luminescent efficiency.
Luminous efficiency may depend on the driving voltage of the light emitting diode, the emission dopant concentration in the light emitting diode, and the life span of the excitons. Accordingly, it may be advantageous, for example, to control the concentration of the emission dopant in order to lower the driving voltage of the light emitting diode and, thereby, increase the luminous efficiency. It may also be advantageous to control the life span of the excitons and their diffusion distance within the EML to further increase the luminous efficiency.
The color of light emitted from the light emitting diode may depend on the specific combination of light emitting layers in the light emitting diode. In particular, a light emitting diode capable of exhibiting natural full-color, i.e., capable of forming pure red, green, and blue light colors, may require a specific configuration of light emitting layers.
For example, in a conventional color conversion matrix (CCM) method, a light emitting diode may include a cyan emission source as a light source with separate red, green, and blue conversion layers to absorb the cyan light and emit red, green, and blue colors, respectively. However, the cyan emission material may have a low lifespan and luminous efficiency, while the overall CCM method may require a complicated manufacturing process.
In another exemplary conventional method, red light emission may be achieved by combining either blue and green organic light emitting diodes or cyan and green organic light emitting diodes having a blue color filter. However, both combinations provide low efficiencies due to insufficient red and blue color emissions, respectively. Further, if the light emitting diodes are inorganic, they may require high voltage application and complicated driving circuits, thereby rendering the manufacturing process long and complicated, and provide low efficiency and luminance as compared to the organic light emitting diodes.
Accordingly, there remains a need to improve the structure of the full-color EL display device in order to provide a device capable of providing a single-color light without forming separate red, green, and blue emission layers having different life spans. More importantly, there exists a need for a full-color EL display device exhibiting pure blue, green, and red coordinates with improved luminescent efficiency.